The present invention relates to a method of and apparatus for grinding a workpiece consisting of a thin plate-like member such as a silicon wafer.
Basic arrangement of conventional surface grinder for grinding a workpiece of thin plate-like member can be sorted into four types as shown in FIGS. 1a to 1d according to the direction of the axis of the spindle which supports and drives the grinding wheel and the shape of the table mounting the work. Namely, FIG. 1a shows a surface grinder of vertical shaft and rotatable circular table type having a spindle 1a whose axis extends vertically. A grinding wheel 2a attached to the lower end of the spindle 1a rotates in the direction of the arrows with its side surface consisting the grinding surface. A circular table 4a carrying a workpiece 3a having the surface to be treated directed upwardly and held horizontally is adapted to rotate in the direction of the arrow. FIG. 1b shows a surface grinder of vertical shaft and reciprocatable angular table type in which a grinding wheel 2b is fixed to the lower end of a spindle 1b having a vertical axis such that its side surface constitutes the grinding surface and is rotated in the direction of the arrow. An angular table 4b carries a workpiece 3b such that the surface to be treated is held horizontally and directed upwardly and is adapted to move reciprocatingly in the horizontal direction. FIG. 1c shows a surface grinder of horizontal shaft and rotatable circular table type consisting of a spindle 1c having a horizontal axis, a grinding wheel 2c fixed to one end of the shaft 1c and rotatable in the direction of the arrow with its peripheral surface constituting the grinding surface, and a circular table 4c which rotates in the direction of the arrow while carrying a workpiece 3c such that the surface to be ground is held horizontally and directed upwardly. FIG. 1d shows a surface grinder of horizontal shaft and reciprocatable angular table type having a spindle 1d having a horizontal axis, a grinding wheel 2d fixed to one end of the spindle and rotatable in the direction of the arrow with its peripheral surface constituting a grinding surface, and an angular table 4d adapted to move reciprocatingly in the horizontal direction while carrying a workpiece 3d such that the surface to be ground is directed upwardly and held horizontally.
In any one of the above-mentioned four types of surface grinders, the feed of the grinding wheel 2a, 2b, 2c and 2d has been achieved relatively to the workpiece 3a, 3b, 3c and 3d by horizontal movement of the table 4a, 4b, 4c and 4d. Namely, only the table is moved while the grinding wheel rotates at a stationary position during the grinding.
Therefore, the following problems are encountered in the grinding of a workpiece by the above-explained conventional surface grinders.
In the conventional surface grinders, as schematically shown in FIG. 2, a spindle head 7 is adapted to slide along the vertical guide surface 6' formed on a column 6 fixed to a bed 5. A spindle 8 is rotatably mounted in the head 7 with the axis thereof extended vertically. A grinding wheel 9 is fixed to the lower end of the spindle 8 and is adapted to rotate in the direction of the arrow while keeping its grinding surface horizontal. A table 10 is mounted horizontally and rotatably on the bed 5 and carries at its upper surface a workpiece 11. In this type of surface grinder, the grinder as a whole is symmetrically constructed across the spindle 8. Therefore, although the heat generated in the spindle head 7, as a result of rotation of the spindle 8, is transmitted to the column 6 through the guide surface 6', the the column 6 is heated so that the column 6 is not thermally distorted in the fore and aft direction. However, as to the right and left direction, the column 6 is heated only at the side thereof contacted by the spindle head 7, so that the column 6 is thermally distorted and bent as illustrated in an exaggerated manner. This in turn declines the grinding surface of the grinding wheel 9 through the spindle head 7 and the spindle 8. In this case, when the feed is made by moving the table 10 in the horizontal direction, the work 11 is ground following the declined grinding surface, so that the precision of grinding is deteriorated considerably. Therefore, it has been proposed to correct the levelness of the declined grinding surface of the grinding wheel by dressing. As schematically shown in FIG. 3, the most ordinary method for effecting the dressing is to rotate the central portion of the grinding surface of the grinder wheel 9 in contact with a dresser 12 fixed to the table 10 which is kept still during dressing. With this method, however, it is not possible to recover the perfect levelness of the grinding surface of the grinding wheel because the upper end portion of the dresser 12 effects the dressing following the declined grinding surface of the grinding wheel 9.
FIG. 4 shows an example of a grinding apparatus of the type described heretofore, applied to the grinding of silicon wafer. In this case, a rough grinding wheel 13a and a finish grinding wheel 13b are arranged in a side-by-side relation, while two workpieces 15 are mounted on a table 14 so that these workpieces 15 are simultaneously ground by respective grinding wheels.
According to this arrangement, the rough grinding performed by the rough grinding wheel 13a and the finish grinding performed by the finish grinding wheel 13b are made along paths a and b of the same pattern. This inconveniently promotes the tendency of hair crack in the workpieces.
FIG. 5 shows another example of application of the surface grinder in which a plurality of workpieces 15a, 15b and 15c are mounted on a table 14, so that, while workpieces 15a is being ground, another workpiece 15b, previously ground is taken out of the table 14 or still another workpiece 15c to be ground, is attached to the table 14. In this arrangement, it is necessary to lower the speed of movement of the table 14 to a level which is suitable for attaching and detaching of the workpieces 15b, 15c, resulting in a reduction of the grinding efficiency. According to this grinding method, therefore, the table 14 is temporarily stopped when the grinding is finished with the workpiece 15a to permit attaching and detaching of the workpieces 15c and 15b to and from the table 14. Consequently, the efficiency of the whole grinding operation is lowered disadvantageously.